The previous administrator, Charles F. Bolden Jr., stepped down at the end of President Obama’s term. Mr. Lightfoot, who was associate administrator, the third-highest ranking position at NASA, took over as acting administrator.
In September, President Trump nominated Representative Jim Bridenstine, Republican of Oklahoma, to be the next administrator. But the Senate has yet to vote to confirm Mr. Bridenstine.
All 49 Democrats in the Senate appear unified in opposition, in part because Mr. Bridenstine gave a speech disparaging climate change several years ago. Senator Marco Rubio, Republican of Florida, also has expressed doubts about Mr. Bridenstine.
The space agency’s No. 2 position, deputy administrator, is vacant, and the Trump administration has yet to nominate anyone to fill the post. Steve Jurczyk, formerly the associate administrator for space technology, was named in late February to temporarily fill Mr. Lightfoot’s previous job.
NASA is also lacking a chief of staff.
Still, the agency continues to explore the cosmos. Next month, it is scheduled to launch the Transiting Exoplanet Survey Satellite, or Tess, a space telescope that is to search for planets around other stars.
The InSight spacecraft is due to launch in May for Mars, and the Parker Solar Probe is expected to begin a yearslong mission to study the sun in July.
In August, Boeing and SpaceX both plan to perform their first tests of new capsules that are to carry NASA astronauts into orbit, and the Osiris-Rex spacecraft will rendezvous with an asteroid, Bennu, after a two-year journey.
Mr. Lightfoot’s 406-day term as acting administrator is by far the longest NASA has operated without a permanent leader, eclipsing the 176 days that passed at the start of the Obama administration before Mr. Bolden was confirmed.